The Spanish do not normally eat much breakfast. The typical desayuno consists of a slice of buttered toast, (or a 'curasan-croissant, or "maddalenas-little spongy cakes), a cup of café con leche, perhaps a glass of orange juice.
My American metabolism didn't thrive on such light fare. So, it was a welcome experience when we could have a heartier breakfast. This photo was taken in the Restaurante Santelmo, in Arenas de Cabrales, Asturias. (They are used to English and American tourists)
Typically, the eggs have very orange yolks, the bacon is barely cooked, but tastes great. If you order huevos con jamón, the ham is "serrano", the prosciutto-like ham favored in Spain

Another option, offered in some bars in the morning, is to order a "sandwich mixto con huevo". This is a grilled ham and cheese sandwich, with a hole cut in the top that holds a fried egg.
Cereals and porridge were never seen in our travels. That might be available in more deluxe hotel restaurants.
Another great place for breakfast is the Panadería Vejo in Reinosa, Cantabria. See below.